1. Field of the Invention
This invention relates to a method of forming gaskets by injection and compositions for use therein, particularly acrylic compositions and compositions which incorporate an expansion system.
There is a demand for gasketing compositions which can be injected between two components, for example in an engine or other machine, and which will then polymerise in situ. The two components will have been assembled face-to-face, normally with two flat faces such as on mating flanges. A gasketing composition will then be injected into a groove in one of the faces, so that the composition flows along the groove until it is distributed throughout the length of the groove, which forms a sealing path around the area of the mating surfaces. The composition fills the groove and flows into microscopic gaps between the two contiguous surfaces. The composition then cures in situ to form the gasket. Such gasketing compositions should be usable not only in industrial manufacturing plants but also in repair shops where engines and the like are serviced.
2. Description of the Related Art
It has been proposed to use single-component polysiloxane block copolymers for in situ gasketing. However, polysiloxane block copolymers have a high viscosity even prior to curing--a viscosity of 500,000 to 1.times.10.sup.6 mPas being normal. This high viscosity results in difficulties during application of the uncured composition by injection. In particular, due to the poor flow characteristics, coverage of large areas is time consuming, application of the composition to narrow, deep grooves is difficult, and furthermore high pressures are required to ensure uniform surface coverage. Due to the nature of gasketing, fluid-tight seals are required and hence uniform coverage is essential. A further disadvantage of polysiloxane block polymers is that polymerization in-situ requires a relatively long cure period. In summary, application of polysiloxane copolymers requires high pressures, sophisticated equipment and prolonged cure periods. High pressures are not available in repair shops and even in industrial premises they may be undesirable due to the risk of misuse or of deformation of components between which a gasket is being formed.
U.S. Pat. No. 4,731,982 Grant et. al. assigned to Loctite (Ireland) Limited describes a method of sealing leaking gas pipe joints by injecting a catalyzed acrylic monomer composition into the leaking joint and curing the composition in the joint to seal the leak. Suitable injectable sealants described therein are mixtures of long chain mono-acrylic esters, hydroxy functional acrylic esters, acid monomers, plasticisers and polyfunctional acrylic esters. The viscosity of the compositions is preferably no greater than 100 cps (mPas), and suitably less than 50 cps (mPas). Although no cure time is specified therein, the Examples mention cure times of 24 hours (Example 1), 1 day, 2 days or 7 days (Example 2) or gellation times of 40 minutes (Example 3) or 10 minutes (Example 5). A composition with a very low viscosity and a long cure time would not be suitable for use in a gasketing application, where significant gaps occur between engineering components and a composition with a viscosity of less than 100 mPas would flow out through the gaps before gelation occurred.
U.S. Pat. No. 4,772,031 Poppo describes a sealant system for repairing leaking pipes comprised of an elastomeric modified acrylic adhesive, exothermically curable upon activation by a catalyst, and a blowing agent. A two compartment container is used to hold, mix and inject the sealant into a mould surrounding the leaking pipe. One compartment of the container is filled with a mixture of the elastomeric modified acrylic, a filler, a plasticizer and a blowing agent such as p.p-oxybis (benzenesulfonyl hydrazide). The second compartment is filled with a free radical polymerisation catalyst such as benzoyl peroxide. After mixing the components, the sealant is pumped into the mould cavity prior to significant reaction of the acrylic. The mould is sealed and the reaction of the acrylic proceeds to completion concomitant with a rise in temperature, decomposition of the blowing agent and an increased pressure within the mould. The internal pressure thus generated forces the sealant into the interstices of the pipe. The sealant upon complete curing and solidification is bonded to the surface of the pipe.
The Poppo Patent is concerned with the formation of a sealing sleeve around a pipe joint and is therefore dealing with much larger volumes of composition than would be used for injection gasketing. The mould cavity 66 described by Poppo has a large volume and is sealed at its edges by rubber gaskets 64. Therefore the viscosity of the uncured composition is not of significance to the Poppo system and is not mentioned in the Patent. Likewise the cure time is not mentioned, although the test described at column 4 line 67-column 5 line 6 not carried out until the seal had cured for 12 hours. The Poppo Patent does not contain any teaching about the special requirements of injection gasketing.
U.S. Pat. Nos. 4,582,551 Parkes et. al., and 4,673,449 Webb et. al., describe methods of sealing pipe joints or leaks with anaerobic monomers. These are low viscosity liquids.
U.S. Pat. No. 4,439,600 Moran assigned to Loctite Corporation describes cure-to-elastomer compositions which, depending on their formulation, may be used as sealing compounds, gasketing compounds, adhesives and the like. The cure-to-elastomer composition comprises (A) a moderate-to-long chain di or polyfunctional prepolymer having vinyl reactive ends, (B) a cross-link controlling material which is (i) reactive with said moderate-to-long chain di- or polyfunctional prepolymer, and (ii) soluble in or miscible with the moderate-to-long chain prepolymer and (C) a free radical polymerization initiator. A surface activator may be used to increase the speed of cure but is not required (column 11 lines 57-column 12 line 8).
The compositions of the Moran Patent are said to be useful as resilient adhesives, inter alia in gaskets. However there is no mention of gaskets formed by injection and no consideration of the problems involved in injection gasketing.
In general, polymerization of polymer precursors results in a decrease in volume. This volume reduction is of particular importance in gasketing applications wherein a decrease in volume upon polymerization in-situ may result in a poor unreliable seal.
U.S. Pat. No. 4,772,031 Poppo (already mentioned) describes a pipe sealant composition which contains a heat-activated blowing agent. The blowing agent decomposes releasing gaseous by-product during exothermic reaction of the activated acrylic adhesive. The sleeve seal produced by Poppo (see FIG. 1 thereof) is exposed on its external surface and therefore Poppo does not provide any teaching concerning shrinkage in a gasket formed between two mating components. The bulk of the composition used by Poppo is sufficiently great for an exothermic reaction to be effective in decomposing the blowing agent; as shown in FIG. 7 of Poppo, the temperature in the reaction mixture rises to substantially in excess of 100.degree. C. and is maintained at that level for several minutes. However in a gasketing composition extending along a groove between two mating components the dissipation of any heat generated in the reaction mixture would be too rapid for the decomposition of a heat-activated blowing agent to be initiated and/or sustained.